A test tape unit is known from EP-A 1 739 432 of the applicants. In that case areas for determining the distance travelled are provided in the form of raster strips between the test fields which are scanned during the tape movement to determine the tape transport path wherein the corresponding signal flank alternation is added up in a counter so that the counter reading is proportional to the distance travelled by the tape. From this one can also derive a position detection, but only the rapid and exact positioning of test fields at the designated measurement site is referred to in this connection. The scanned distance increments can not, however, be differentiated from one another and hence this indirect method of position determination requires that an unnoticed change in the tape position has not taken place.